Pixel sensor cells (e.g., CMOS imagers) can suffer from background noise due to thermal carrier generation, i.e., stray light. To compensate for such unwanted noise, i.e., stray light, conventional pixel sensor cells require “dark pixels”. The dark pixels are formed by an aluminum light shield over the dark pixels, and surrounding the peripheral of the active devices, e.g., photo cell collector regions. In conventional structures, the aluminum light shield is formed on an upper wiring layer, e.g., third or fourth wiring level, over the dark pixels. In theory, the aluminum shield blocks light from hitting the transistor structures, i.e., dark pixels, rendering the dark pixels insensitive to photons. This, in turn, allows these dark pixels to generate an accurate reference signal. The reference signal can then be subtracted from the total charge signal to allow a sensor to measure the dark level offset used in downstream signal processing to perform auto black level calibration, for example.
Under normal circumstances, the dark pixels do not respond to light; however, the dark pixels are in close proximity to the active pixels, i.e., photo cell collector regions, or the outer bounds of the chip (including the first two lines out) such that they can scavenge signals depending on light intensity and wavelength. More specifically, due to the location of the aluminum shield in the upper wiring layer, it is common for incident stray light to strike the dark pixels. This is mainly due to the location and size of the light shield and angle of attack of the incident light, which can enter beneath the light shield. The dark signals therefore will not represent the true dark signal. Also, it is known that the aluminum shield adds material complexities to the fabrication process, in addition to using valuable real estate on the chip, that can otherwise be used for other wiring and/or device fabrication.
Another approach to block light from striking the dark pixels is to use a black resist. However, a black resist is extremely hard to process. For example, the dark resist covers alignment marks on the wafer. That is, a dark resist interferes with alignment and overlay measurements during processing. Also, the black resist is difficult to image through, and is very expensive from a material standpoint. Accordingly, it is not practical to use a black resist.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.